I didn't know, so I did some research for you and found this: This has evolved from the Tudor custom where guests would throw shoes at the newlywed couple. It was considered lucky if they or their carriage were hit. http://www.hbs-e.com/wedding_album/wedding_customs_1.htm but as usual, there is always someone who will have another idea: In times past, leather was believed to ward off evil spirits, and so old leather shoes were thought to keep the newlyweds safe. In times past, leather was believed to ward off evil spirits, and so old leather shoes were thought to keep the newlyweds safe. And here are some other possibilities: The custom of tying shoes to the car bumper stems way back to Ancient Egypt, where a father would give the groom his daughter’s sandal, marking that an exchange had taken place. In Anglo-Saxon marriages, the groom tapped the bride on the head with the shoe to demonstrate his authority. Since feet were considered a phallic symbol, guests threw old shoes at the bride to promote fertility. Eventually, shoes and cans were tied to the departing automobile. The noise was believed to keep evil spirits away http://www.bedbathandbeyond.com/regShowWWTip.asp?order_num=1&tip=wedding_getawaycar&tiptype=Couple The answer then: seems no one really knows, but everyone has an idea. LOL (and the cans,making noise would also ward off the spirits too, I suppose)

Also, to give spectators and guests of the wedding a good laugh and often the couple themselves-they can go off to their honeymoon with a smile on their face (and to embarass them). It also alerts strangers that they are newly weds!